The chick-a-dee call is built from discrete note types delivered in a fixed order, with the terminal dee notes variable in number. Playback and presentation experiments found that the count is not random. Presenting perched raptors of different sizes produced systematically more dee notes for smaller predators, which are the manoeuvrable ones capable of catching a bird the size of a chickadee in dense cover.
The signal is read by other birds. Playback of calls with more dee notes draws stronger mobbing responses, and the call is heeded by other species that forage in the same mixed winter flocks, including nuthatches. This makes it one of the clearer demonstrations of graded, information-bearing alarm signalling outside primates.
The second thing chickadees do to survive northern winters is drop their body temperature at night by roughly ten degrees Celsius in regulated nocturnal hypothermia, cutting the cost of staying warm. They also cache thousands of food items and recover them, with the hippocampus enlarging seasonally in association with that spatial memory load.